Field of the Invention
The present invention relates to a method and an apparatus for controlling a CVVT engine of an intermediate phase, and more particularly, to a method and an apparatus for controlling a CVVT engine of an intermediate phase in which operational properties of a plunger in low-temperature regions are improved.
Description of Related Art
Generally, an internal combustion engine is a device for generating power by inhaling air and fuel from an outside thereof and burning them in a combustion room and is provided with intake valves for inhaling air and fuel into the combustion room and exhaust valves for discharging explosion gas that is exploded in the combustion room.
The intake valve and exhaust valve as described above are opened and closed depending on the rotation of a cam shaft that rotates in link with a crank shaft, in which the optimum opening and closing times of the intake valve and exhaust valve may be varied depending on the rotation number of an engine, loads thereon and the like.
In order to determine the optimum opening and closing time of the intake valve and exhaust valve, a device has been developed to properly control the opening and closing time of the valves in accordance with a driving situation of an engine by allowing the cam shaft to be displaced within a set range without determining whether the cam shaft is rotated by only the rotation of the crank shaft, which refers to VVT (Variable Valve Timing).
A Continuous Variable Valve Timing (hereinafter referred to as “CVVT”) is a kind of this VVT, and is has a configuration to control the valve timing to a random value within a set range.
The CVVT system has such advantages that the opening and closing time of the valves is controlled depending on rotation numbers of an engine and thereby improves fuel efficiency, reduces the amount of discharging gas, increases torque at a low speed, and improves output.
That is, a pumping loss is reduced by increasing valve overlaps between the intake valve and exhaust valve and thereby improves fuel efficiency, the amount of discharging gas is reduced by optimizing the valve overlaps depending on an engine condition and re-burning non-burnt gas through internal EGR (Engine Gas Recirculation), and the torque at a low speed and output of the engine are improved by optimizing the intake valve timing depending on the engine condition and increasing volume efficiency.
Recently, the CVVT system of an intermediate phase, which has improved the problems regarding response property and operational region limitation of an existing CVVT system, has been actively developed.
In the CVVT of an intermediate phase, the position of a cam is controlled at an intermediate location, not at maximum retard (intake) and at maximum advance (exhaust), and thus the response property is prompt and the utilizing region of the cam is widened, thereby improving fuel efficiency and reducing the amount of the discharging gas.
The CVVT system of an intermediate phase has been actively developed in the field of new engine, however, there are complicated internal fluid passages and precise control of Oil Control Valve (OCV) is required.
In the CVVT system of an intermediate phase, a plunger which is controlled by DUTY value of Pulse Width Modulation (PWM) as an electric signal controls a cam by controlling the fluid passage while the plunger moves.
As described above, the CVVT system of an intermediate phase controls a CVVT using not only oil force but also cam torque so that the CVVT can be controlled at a low temperature where oil pressure is low while a vehicle starts-on. However, in a case where an engine is at a low temperature, the oil pressure is low and also mechanical friction of components of the engine is increased so that the control for fuel efficiency, performance, and decreasing of the amount of discharging gas is required for operating the CVVT.
The information disclosed in this Background of the Invention section is only for enhancement of understanding of the general background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art already known to a person skilled in the art.